1. Field of the Invention
The present invention relates to thick film heaters comprising a heating element of electrically resistive thick film circuitry, and more specifically to a heater applied directly to a target object.
2. Description of Prior Art
It is often necessary to heat certain objects (“the target object”) for a variety of applications, and it has long been known to accomplish this task with electrical heaters using heating element of an electrically resistive circuit to generate heat. In more recent years it has been known to use heaters with a heating element made of a thick film circuit. It has also been known to use flexible heaters made of two layers of silicon rubber with a wire circuit heating element disposed between the layers. The flexible heater is then placed around the target object. In other applications cartridge heaters comprising a cylindrical metal sheath with a wound heating element disposed therein, are inserted into bores drilled in the target object.
All of these prior heating techniques have serious drawbacks and limitations however. This is particularly true in applications where the target object is used in very low temperatures, for instance 77K, which is the temperature of liquid nitrogen.
For instance, in a cryogenic pump a cartridge heater is conventionally used to heat absorbent for trapping gas molecules and to regulate its temperature to assure proper operation of the pump. There are several limitations to this heating method. Because of the bulk of the heater, there is some distance between the heater and the absorbent to be heated. This longer heat transfer path means longer heat up times, which is compounded by the large thermal mass of a cartridge heater, the additional radiation heat loss, and the limitation on power density (heat flux) when the heater is so distanced from the target. Furthermore, a cartridge heater requires a high precision intermediate thermal conducting layer to improve the contact between the heater and the component. This additional layer (often made of a precious metal) adds significant cost and labor to the pump.
As another example, a DNA analyzer contains a cup holder, which holds plastic cups containing liquids for enzyme reactions to proceed. This cup holder must be heated from extremely low temperatures, and is typically heated using a silicone rubber heated (etched foil type) bonded to the cup holder with an adhesive. The bonding process is very labor intensive and often results in the production of gas bubbles in the adhesive layer. These gas bubbles are poor heat conductors and therefore create zones of localized overheating and uneven temperature distribution overall. These zones also result in delamination of the heater (because of the different zones of thermal expansion) and in many situations, heater failure. The silicone rubber heater suffers from power density limitations that usually limit the heater to 20 W/m2 (3.1 W/cm2).
Many of the above limitations could be overcome, in theory, with the use of thick film heater technology. The thick film resistive circuit could be printed directly on the target object. Unfortunately, thick film heating circuits made of silicone based inks crack after several cycles at such extremely low temperatures, rendering them useless. It is also known to use other polymer-based thick film inks (e.g. epoxy based), but when used at low temperatures, these circuits display gradual changes in resistance with heat cycling. The change in resistance naturally means a change in power density of the heater (assuming constant voltage) which is unacceptable in these applications.
It is thus an object of the present invention to provide a thick film heater integrated with a target object to be heater.
It is a further object of the present invention to provide a thick film heater that can withstand operation in extremely cold ambient temperatures.
It is yet another object of the present invention to provide a novel method or preparing such a thick film heating circuit.
Other objects of the invention will become apparent from the description of the invention, below.